(CBS News) The 2009 H1N1 pandemic that swept through several continents took the lives of 18,500 people from April of that year through August 2010 - or so health officials said at the time.

A new study reveals that the actual death count from the pandemic caused by the so-called "swine flu" strain could be more than fifteen times higher.

"The study underscores the significant human toll of an influenza pandemic," study author Dr. Fatimah S. Dawood, an epidemiologist at the U.S. Centers for Disease Control and Prevention, said in a news release. "We hope that this work can be used not only to improve influenza disease burden modeling globally, but to improve the public health response during future pandemics in parts of the world that suffer more deaths, and to increase the public's awareness of the importance of influenza prevention".

For the study, published in the June 26 issue of The Lancet, researchers aimed to estimate the global number of deaths tied to H1N1 during the first 12 months the virus circulated in 12 low-, middle- and high-income countries where an outbreak occurred. Lab tests following the pandemic confirmed 18,500 deaths, but as a general rule, the number of lab-confirmed flu deaths is known to be significantly lower than the number of actual deaths because some people may never get diagnosed or go to a hospital.

The new estimates were based only on data from countries that hold information on the number of people who developed flu symptoms, as well as the number of respiratory and heart-associated deaths that occurred during the 2009 H1N1 pandemic.

Using a mathematical model they created, the researchers estimated that there were 284,500 respiratory and cardiovascular deaths (201,200 were respiratory) associated with the 2009 pandemic. The results suggest that 80 percent of the deaths occurred in people younger than 65; typically with seasonal influenza, most of the deaths occur in elderly people. What's more, 51 percent of the deaths occurred in Southeast Asia and Africa - regions that account for 38 percent of the world's population

The global toll however could surpass as many as 575,000 deaths, the researchers said.

"This pandemic really did take an enormous toll," Dawood told Reuters. "Our results also suggest how best to deploy resources. If a vaccine were to become available, we need to make sure it reached the areas where the death toll is likely to be highest."

In an accompanying comment published in the same journal, Dr. Cecile Vibuod, a population studies scientist at the National Institutes of the Health, said the new estimates may be accurate for some countries, but a lack of data from some of the low- and middle-income countries in the study may suggest death rates are even higher.

Viboud wrote, "These results are likely to be refined as more studies from low-income and middle-income regions become available, particularly from China and India, where about a third of the world's population live but where little information is available about the burden of influenza."